The present invention relates generally to an apparatus and a method for using electron beams to microscopically inspect the surface of an object, and more particularly to inspect layers in a semiconductor device.
A variety of methods have been used to examine microscopic surface structures of semiconductors. These have important applications in the field of semiconductor chip fabrication, where microscopic defects at a surface layer make the difference between a functioning and a nonfunctioning chip. For example, holes or vias in an intermediate insulating layer often provide a physical conduit for an electrical connection between two outer conducting layers. If one of these holes or vias is inadequately etched or becomes clogged with foreign material, it will be impossible to establish this electrical connection and the whole chip may fail. Examination of the microscopic defects in the surface of the semiconductor layers is necessary to ensure proper functioning of the chips.
Electron beams have several advantages over other mechanisms to examine samples. Light beams have an inherent resolution limitation of about 100 nm-200 nm, but electron beams can investigate feature sizes as small as a few nanometers. Electron beams are manipulated fairly easily with electrostatic and electromagnetic elements, and are certainly easier to produce and manipulate than x-rays.
Electron beams in semiconductor defect inspection do not produce as many false positives (elements inaccurately identified as defective) as optical beams. Optical beams are sensitive to problems of color noise and grain structures whereas electron beams are not. Oxide trenches and polysilicon lines are especially prone to false positives with optical beams due to grain structure.
A variety of approaches involving electron beams have been utilized for examining surface structure. In low-voltage scanning electron microscopy (SEM), a narrow beam of primary electrons is raster-scanned across the surface to emit secondary electrons. If the primary electrons in the beam of scanning electron microscopy are near a particular known electron energy (called E2), there is a minimal corresponding charge build-up problem associated with SEM, and the surface of the sample remains relatively neutral. However, raster scanning a surface with scanning electron microscopy is slow because each pixel on the surface is collected sequentially. Moreover, a complex and expensive electron beam steering system is needed to control the beam pattern.
Another approach is called Photo-Electron Emission Microscopy (PEM or PEEM), in the which photons are directed at the surface of the sample to be studied, and by the photoelectric effect, electrons are emitted from the surface. On an insulating surface, the emission of these electrons, however, produces a net positive charge on the sample surface since there is a net flux of electrons from the surface. The sample continues to charge positively until there are no more emitted electrons, or electrical breakdown occurs. This charge build-up is especially problematic when imaging insulator materials.
Another method of examining surfaces with electron beams is know as Low Energy Electron Microscopy (LEEM), in which a relatively wide beam of low-energy electrons is directed to be incident upon the surface of the sample, and electrons reflected from the sample are detected. However, LEEM suffers from a similar charge build-up problem since electrons are directed at the sample surface, but not all of the electrons are energetic enough to leave the surface, which repels further electrons from striking the sample, resulting in distortions and shadowing of the surface.
Several prior art publications have discussed a variety of approaches using electron beams in microscopy, but none have determined how to do so with parallel imaging, while at the same time reducing or eliminating the charge build-up problem. One of these approaches is described by Lee H. Veneklasen in xe2x80x9cThe Continuing Development of Low-Energy Electron Microscopy for Characterizing Surfaces,xe2x80x9d Review of Scientific Instruments, 63(12), December 1992, Pages 5513 to 5532. Vaneklasen notes generally that the LEEM electron potential difference between the source and sample can be adjusted between zero and a few keV, but he does not recognize the charging problem or propose a solution to it.
Thus, there remains a need for methods and apparatus that utilize electron beams to investigate sample surfaces, while minimizing charge build-up problems and increasing the speed of examining sample surfaces.
Accordingly, the present invention addresses the above problems by providing apparatus and methods for parallel imaging. In general terms, a first beam is used to generate an image of a relatively wide area of a sample. Parallel imaging is accomplished by using a relatively wide beam. A second beam having a lower landing energy than the first beam may be used in order to reduce positive charge build up on the sample that may result from the first beam.
In one embodiment, an apparatus for inspecting a sample is disclosed. The apparatus includes at least a first electron beam generator arranged to direct a first electron beam having a first range of energy levels toward a first area of the sample and a second electron beam generator arranged to direct a second electron beam having a second range of energy levels toward a second area of the sample. The second area of the sample at least partly overlaps with the first area, and the second range of energy levels are different from the first range such that charge build up caused by the first electron beam is controlled. The apparatus further includes a detector arranged to detect secondary electrons originating from the sample as a result of the first and second electron beam interacting with the sample. In a preferred embodiment, the first electron beam has a width appropriate for parallel multi-pixel imaging, and the first and second electron beam generator are arranged to concurrently produce the first and second beams.
In another embodiment, a method for controlling charging of a surface while exposing the surface to a beam of charged particles is disclosed. The surface is exposed to a first set of electrons in a first beam with the first set of electrons having energies within a first range. The surface is exposed to a second set of electrons in a beam with the second set of electrons having energies within a second range, different from the first range, wherein the second range of energies is predetermined to provide electrons from the second set which land on the surface to reduce positive charge present on the surface. In a preferred embodiment, the surface is concurrently exposed to said first set of electrons and said second set of electrons.
The present invention has several advantages. For example, the apparatus and methods allow imaging of a large number of pixels in parallel on a detector array, and thereby has the properties of being faster and lower in noise than conventional Scanning Electron Microscopes and methods. Additionally, electron beam scanning systems are not required, and the electron beam current densities are not as high so that the probability of damaging sensitive samples is lessened.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures which illustrate by way of example the principles of the invention.